This invention relates to improved techniques for determining position by radio, more particularly for determining position instantaneously, from carrier-wave phases of radio signals received from transmitters having different carrier-wave frequencies. In a preferred embodiment of the invention, these carrier waves also have random phases, and the position of a receiver is determined by reference to the phases of signals received by another, reference, receiver. The invention includes resolving, or reducing, the position ambiguity stemming from the integer-cycle ambiguity inherent in an observation of the phase of a periodic wave such as a carrier wave. Resolving ambiguity enables practically instantaneous position-determination, which is a substantial improvement over prior-art techniques based on sustained, continuous, carrier phase tracking.
In U.S. Pat. No. 4,667,203 one of the present inventors (Counselman) discloses methods and systems for determining position from carrier-wave phases of radio signals received from a plurality of transmitters aboard earth-orbiting satellites such as those of the Global Positioning System (GPS). The disclosure of Counselman ""203 includes methods and systems for resolving position ambiguity caused by the integer-cycle ambiguity of carrier phase. In a preferred embodiment of Counselman ""203, the transmitted signals have random phases, and the position of a receiver is determined by reference to the phases of signals received at another, reference, receiver. However, Counselman ""203 requires the transmitted signals to have suppressed carriers and/or wide, overlapping, spectra.
Counselman ""203 does not teach or render obvious a technique for determining position from carrier waves transmitted with different frequencies. Counselman ""203 does not teach or render obvious an instantaneous positioning technique. On the contrary, Counselman ""203 teaches determining position by combining observations spanning a significant period of time, such as several thousand seconds.
In the Russian GLONASS system, which is otherwise very similar to GPS, different satellites transmit signals with slightly different implicit carrier frequencies. (Both the GLONASS and the GPS satellites transmit substantially overlapping spread-spectrum signals whose carriers are actually suppressed.) These slight carrier-frequency differences are intended to facilitate the separation of different satellites"" signals in a receiver. An instantaneous radiopositioning method using combined GPS and GLONASS observations that includes resolving implicit carrier-phase ambiguities despite the GLONASS frequency differences is disclosed in a paper entitled xe2x80x9cSingle-epoch integer ambiguity resolution with GPS-GLONASS L1 data,xe2x80x9d by M. Pratt et al., appearing in the Proceedings of the Institute of Navigation Annual Meeting in Albuquerque, N. Mex., June 1997, pp. 691-699.
In an article entitled xe2x80x9cMiniature Interferometer Terminals for Earth Surveying (MITES)xe2x80x9d, appearing in Bulletin Geodesique, Volume 53 (1979), pp. 139-163, by Charles C. Counselman III and Irwin I. Shapiro, there is proposed a system for determining position by measuring carrier-wave phases of multi-frequency radio signals received from a plurality of earth-orbiting satellites. The reason for having multiple frequencies is to resolve ambiguity and determine position instantaneously. However, the MITES scheme requires each one of the plurality of transmitters to emit the same multiplicity of different-frequency carrier waves. (Small frequency shifts and/or modulation are used to mitigate interference, as in the above-mentioned GLONASS system.) In other words, although Counselman and Shapiro teach the use of multiple frequencies, they teach that the multiple frequencies must be transmitted by each single transmitter; and that all transmitters should transmit the same frequencies.
The idea that all transmitters utilized in a radio-positioning technique should transmit the same or nearly the same frequencies is fundamental to many radiopositioning systems, including Loran and Omega (discussed below) in addition to GPS, GLONASS and MITES (discussed above). It is desired for all transmissions to be the same frequency band, or xe2x80x9cchannel,xe2x80x9d not merely to conserve spectrum, but fundamentally to facilitate differential measurements, i.e., measurements of differences between signals received from different transmitters.
The Loran system is described in an article by W. O. Henry, entitled xe2x80x9cSome Developments in Loran,xe2x80x9d appearing in the Journal of Geophysical Research, vol. 65, pp. 506-513, February 1960. The current version of Loran, known as Loran-C, employs several-thousand-kilometer-long chains of synchronized transmitters stationed on the surface of the earth, with all transmitters having the same implicit carrier frequency, 100 kiloHertz, but with each transmitter emitting a unique time-sequence of short pulses. This sequence, which includes polarity reversals of the pulses, enables a receiver to distinguish between signals from different transmitters. A suitable combination of observations of more than one pair of transmitters can yield a determination of the receiver""s position on the surface of the earth. Basically, a receiver observes the difference between the times of arrival of pulses from a pair of transmitters. Since the transmitters are synchronized, a time-difference-of-arrival (TDOA) observation implies that the receiver is located somewhere along a particular hyperbolic curve having vertices at the transmitters. (The locus of points having a given difference between their distances from two vertices is an hyperbola.) Observing TDOA for additional pairs of transmitters provides additional hyperbolic constraints on the receiver""s position, and enables a unique position to be determined.
The Omega system is described in an article by Pierce, entitled xe2x80x9cOmega,xe2x80x9d appearing in IEEE Transactions on Aerospace and Electronic Systems, vol. AES-1, no.3, pp. 206-215, December 1965. Omega, like Loran and conventional GPS, is an hyperbolic positioning system. In the Omega system, the phase difference between the radio waves received from different transmitters is measured rather than (principally) the time difference (TDOA) as in the Loran-C system. To facilitate resolution of phase ambiguity, Omega transmitters transmit plural frequencies. However, different transmitters transmit the same frequencies. Again, this is done to facilitate differential measurements.
It is known in the radiopositioning art, i.e., the art of determining position by radio, to utilize signals of opportunity, by which we mean signals emitted by uncooperative transmitters. Typically such transmissions are not intended for positioning; different transmitters operate on wholly different frequencies; they are not synchronous; and their carrier-wave phases are random. Lack of synchronization or instability in time, frequency, and/or phase prevents many radiopositioning methods from being usefully employed.
An example of radiopositioning by utilizing signals of opportunity is determining position by radio direction finding (RDF) observations of commercial broadcast signals in the medium-frequency, amplitude-modulated (AM) broadcast band from about 550 to 1700 kHz. These signals are transmitted for purposes other than positioning, but the transmitters are marked and identified on nautical charts to facilitate their use as radio beacons, for navigation by RDF. Different AM broadcast transmitters within any given region of the country (or world) are assigned to completely separate, disjoint, frequency channels to avoid interference.
Another prior-art radiopositioning technique utilizing signals of opportunity tracks the phases of the carrier waves of signals received from commercial broadcasters in the medium-frequency AM broadcast band. By xe2x80x9ctracking the phase of the carrier wave of a signalxe2x80x9d we mean continuously or effectively continuously measuring the phase of the carrier wave with respect to a local reference oscillator; and keeping track of the time-variation of the measured phase from an initial time when the receiver""s position was known, to a later time when the position is unknown and to be determined. Continuity of tracking is essential. If tracking were interrupted, the phase change during the hiatus would be unknown, so later positions could not be determined without resort to external information. This problem is discussed further in the following section.
In an article entitled xe2x80x9cThe CURSOR Radio Navigation and Tracking System,xe2x80x9d by Peter J. Duffett-Smith et al., appearing in the Journal of Navigation, vol. 45, no. 2, May 1992, pp. 157-165, an AM-broadcast-band carrier phase tracking system called xe2x80x9cCURSORxe2x80x9d is described and the statement is made, xe2x80x9cA drawback of a phase-measuring CURSOR system is its need to track the signals continuously from each radio transmitter. In many applications this is not a problem, but when used for vehicle tracking in cities there are always heavily-shadowed regions such as tunnels, underpasses, and petrol-station forecourts where the signals become too weak to track or disappear altogether. When the vehicle emerges from the shadow, the receivers lock themselves back on to the signals, but there is now an uncertainty equal to an integer number of wavelengths in the measured phases from each transmitter. Hence, the . . . [vehicle""s] true position is no longer known. However, by using many more than the minimum three channels [i.e., broadcast stations], the solutions to the equations are constrained to the extent that only one of them is usually physically possible.xe2x80x9d (Op cit., at the bottom of page 163 and top of page 164.)
Thus, Duffett-Smith et al. disclose that phase-ambiguity can be resolved. Their method is not disclosed in enabling detail. It also seems not to have worked well for them, because they state (in the paragraph beginning at the middle of page 164), xe2x80x9cA further drawback of a phase-measuring CURSOR system is the need to calibrate the system against known positions from time to time.xe2x80x9d Such recalibration is required in a phase-tracking system if only changes in position are being determined, from changes in phase, without an ability to determine position at any single time, i.e., instantaneously.
xe2x80x9cTrackingxe2x80x9d methods such as the one described by Duffett-Smith et al. are intrinsically incapable of instantaneous positioning. By xe2x80x9cinstantaneous positioningxe2x80x9d we mean determining position at an instant, i.e., at a single point in time, as opposed to determining how a position has varied during the extended time interval since continuous tracking began, or last resumed. Tracking is an xe2x80x9cincrementalxe2x80x9d positioning technique, as opposed to an instantaneous positioning technique.
Instantaneous positioning is advantageous, in comparison with incremental positioning, because tracking is subject to being interrupted for many reasons, including both deliberate and accidental reasons. An example of a deliberate reason is to conserve energy by keeping receiver power turned off until a position determination is required. This reason is important for battery-powered equipment. An example of an accidental interruption is that a vehicle carrying a tracking receiver enters a tunnel. Another, serious, disadvantage of incremental positioning is that errors are cumulative.
It is known to combine instantaneous and incremental radiopositioning techniques. For example, in an article entitled xe2x80x9cSynergism of Code and Carrier Measurements,xe2x80x9d by Ron L. Hatch, appearing in the Proceedings of the Third International Geodetic Symposium on Satellite Doppler Positioning, pp. 1213-1231, Feb. 8-12, 1982, Las Cruces, N. Mex., a method of combining incremental position information derived by carrier-phase tracking, with instantaneous position information derived from xe2x80x9ccodexe2x80x9d time-of-arrival measurements, is disclosed. Combining instantaneous and incremental positioning techniques may be advantageous when, as in the case described by Hatch, the incremental technique is more precise than the instantaneous technique.
By xe2x80x9cposition informationxe2x80x9d we mean data relating to position, from which position may be determined. An example of position information is TDOA measurement data. Another example of position information is carrier-phase measurement data. Position, per se, is usually expressed in position coordinates such as latitude and longitude, or northing and easting, relative to some origin or reference.
Most radiopositioning techniques, including those of the present invention, may be xe2x80x9cturned aroundxe2x80x9d; i.e., one may determine the position of a transmitter, instead of or in addition to, the position of a receiver, from measurements of received signals. In relative-positioning techniques, such as in the present invention, determining the position of one receiver relies on having position information from a second receiver. This determination may be performed by combining data from both receivers at either receiver, or in a third place.
It is a general object of the present invention to provide improved techniques for determining position, or navigating, by radio.
A further object of the present invention is to provide improved techniques for instantaneous radiopositioning.
A more specific object is to provide improved techniques for radiopositioning utilizing radio signals received from different transmitters having, respectively, different frequencies.
A still more specific object is to provide improved techniques for radiopositioning utilizing radio signals received from different transmitters having random phases.
A specific object is to provide improved techniques for determining position by utilizing radio signals xe2x80x9cof opportunity,xe2x80x9d i.e., signals emitted by uncooperative transmitters.
Yet another more specific object of the present invention is to provide improved techniques for resolving or reducing ambiguity in determining position by radio.
In a first aspect, the invention which achieves the foregoing objects, is a method of instantaneously determining an unknown position using radio signals from a plurality of transmitters having widely distributed and known positions and a wide range of radio frequencies. The method includes measuring the phases of the radio signals arriving concurrently at an unknown position to obtain a first set of phase-measurement data. The phases of radio signals arriving concurrently at a known reference position is measured almost simultaneously with their arrival at the unknown position to obtain a second set of phase-measurement data. The first and second data sets are combined to determine the unknown position.
In a preferred embodiment the radio signals arrive at the unknown position via ground-wave propagation from the plurality of transmitters which operate independently and whose radio signals are transmitted with random phases. In another embodiment, the first set of phase measurement data refers to a first instant of concurrent-signal-arrival at the unknown position and the second set of phase-measurement data refers to a second instant of concurrent-signal-arrival time at the reference position, and the departure from simultaneity of the first and second instants is determined simultaneously with the determination of the unknown position. It is preferred that position ambiguity be resolved by establishing an ambiguity function and finding the location of the maximum of the ambiguity function.
In another aspect, the invention is a radiopositioning system for instantaneously determining an unknown position. The system includes a plurality of spatially distributed transmitters at known locations, the transmitters transmitting signals at widely distributed frequencies. A first receiver is located at the unknown position and is adapted to receive the signals from the plurality of transmitters and to determine the phases of the signals at the unknown position to generate a first set of phase-measurement data. A second receiver is located at a known reference position adapted to receive the signals from the plurality of transmitters and to determine the phases of the signals at the known reference position to generate a second set of phase-measurement data. Computer apparatus operates on the first and second sets of phase-measurement data to determine the unknown position. In a preferred embodiment, the computing apparatus is programmed to find the location in parameter space of the maximum of an ambiguity function of the sets of phase-measurement data, the location of the maximum being the unknown position. It is preferred that the ambiguity function be a sum, over all of the transmitters, of a periodic function of the phase-measurement data. It is also preferred that the periodic function have a period of one phase cycle. Other appropriate functions are disclosed hereinbelow. It is also preferred that the output of the first and second receivers be a composite signal that is then digitized for processing.